libxl: do not leak spawned middle children

libxl: do not leak spawned middle children

libxl__spawn_spawn would, when libxl__spawn_detach was called, make
the spawn become idle immediately.  However it still has a child
process which needs to be waited for: the `detachable' spawned
child.

This is wrong because the ultimate in-libxl caller may return to the
application, with a child process still forked but not reaped libxl
contrary to the documented behaviour of libxl.

Instead, replace libxl__spawn_detach with libxl__spawn_initiate_detach
which is asynchronous.  The detachable spawned children are abolished;
instead, we defer calling back to the in-libxl user until the middle
child has been reaped.

Also, remove erroneous comment suggesting that `death' callback
parameter to libxl__ev_child_fork may be NULL.  It may not, and there
are no callers which pass NULL.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>